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Can you hear sound through the headphone port? If so then it's possible the speaker wire came loose on the inside. If not, then it's most likely a motherboard problem and you should contact ACER for warranty service.

Actually you need to install chipset drivers first then you shoud try for additiona PCI driver like external sound card.
Try one thing again, go to your www.acer.com site and search driver as per your "Service tag no." not for model no. and download drivers for 'Chipset' and others as per your requirement.